Checking if an object is null in C#

Question

I would like to prevent further processing on an object if it is null.

In the following code I check if the object is null by either:

if (!data.Equals(null))

and

if (data != null)

However, I receive a NullReferenceException at dataList.Add(data). If the object was null, it should never have even entered the if-statement!

Thus, I'm asking if this is proper way of checking if an object is null:

public List<Object> dataList;
public  bool AddData(ref Object data)
    bool success = false;
    try
    {
        // I've also used "if (data != null)" which hasn't worked either
        if (!data.Equals(null))
        {
           //NullReferenceException occurs here ...
           dataList.Add(data);
           success = doOtherStuff(data);
        }
    }
    catch (Exception e)
    {
        throw new Exception(e.ToString());
    }
    return success;
}

If this is the proper way of checking if the object is null, what am I doing wrong (how can I prevent further processing on the object to avoid the NullReferenceException)?

Answer 1

[Edited to reflect hint by @kelton52]

Simplest way is to do object.ReferenceEquals(null, data)

Since (null==data) is NOT guaranteed to work:

class Nully
{
    public static bool operator ==(Nully n, object o)
    {
        Console.WriteLine("Comparing '" + n + "' with '" + o + "'");
        return true;
    }
    public static bool operator !=(Nully n, object o) { return !(n==o); }
}
void Main()
{
    var data = new Nully();
    Console.WriteLine(null == data);
    Console.WriteLine(object.ReferenceEquals(null, data));
}

Produces:

Comparing '' with 'Nully'

True

False

Answer 2

C# 6 has monadic null checking :)

before:

if (points != null) {
    var next = points.FirstOrDefault();
    if (next != null && next.X != null) return next.X;
}   
return -1;

after:

var bestValue = points?.FirstOrDefault()?.X ?? -1;

Answer 3

Your dataList is null as it has not been instantiated, judging by the code you have posted.

Try:

public List<Object> dataList = new List<Object>();
public  bool AddData(ref Object data)
bool success = false;
try
{
    if (!data.Equals(null))   // I've also used if(data != null) which hasn't worked either
    {
       dataList.Add(data);                      //NullReferenceException occurs here
       success = doOtherStuff(data);
    }
}
catch (Exception e)
{
    throw new Exception(e.ToString());
}
return success;

}

Answer 4

The problem in this case is not that data is null. It is that dataList itself is null.

In the place where you declare dataList you should create a new List object and assign it to the variable.

List<object> dataList = new List<object>();

Answer 5

Whenever you are creating objects of class you have to check the whether the object is null or not using the below code.

Example: object1 is object of class

void myFunction(object1)
{
  if(object1!=null)
  {
     object1.value1 //If we miss the null check then here we get the Null Reference exception
  }
}

Answer 6

in C# 7 the best is

if (obj is null) ...

This will ignore any == or != defined by the object (unless of course you want to use them ...)

For not equal you can if (!(obj is null)) (ugly)